/Engine/Source/Runtime/Landscape/Public/LandscapeEdit.h
Replace/merge a layer (across all edit layers and the main runtime layer)
void ReplaceLayer ( [ULandscapeLayerInfoObject](API\Runtime\Landscape\ULandscapeLayerInfoObject) * FromLayerInfo, [ULandscapeLayerInfoObject](API\Runtime\Landscape\ULandscapeLayerInfoObject) * ToLayerInfo )